Re: [PATCH 5/6] slab: validate slab before using it in alloc_single_from_partial()

Vlastimil Babka
2025-09-12T11:34:15Z

On 9/12/25 12:48, Harry Yoo wrote:
> On Thu, Sep 11, 2025 at 07:02:38PM +0200, Vlastimil Babka wrote:
>> We touch slab->freelist and slab->inuse before checking the slab pointer
>> is actually sane. Do that validation first, which will be safer. We can
>> thus also remove the check from alloc_debug_processing().
>>
>> This adds a new "s->flags & SLAB_CONSISTENCY_CHECKS" test but
>> alloc_single_from_partial() is only called for caches with debugging
>> enabled so it's acceptable.
>>
>> In alloc_single_from_new_slab() we just created the struct slab and call
>> alloc_debug_processing() to mainly set up redzones, tracking etc, while
>> not really expecting the consistency checks to fail. Thus don't validate
>> it there.
>>
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> ---
>> mm/slub.c | 17 ++++++++---------
>> 1 file changed, 8 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 909c71372a2f542b6e0d67c12ea683133b246b66..93df6e82af37c798c3fa5574c9d825f0f4a83013 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -1651,11 +1651,6 @@ static noinline bool alloc_debug_processing(struct kmem_cache *s,
>> struct slab *slab, void *object, int orig_size)
>> {
>> if (s->flags & SLAB_CONSISTENCY_CHECKS) {
>> - if (!validate_slab_ptr(slab)) {
>> - slab_err(s, slab, "Not a valid slab page");
>> - return false;
>> - }
>> -
>> if (!alloc_consistency_checks(s, slab, object))
>> goto bad;
>> }
>> @@ -2825,15 +2820,19 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
>>
>> lockdep_assert_held(&n->list_lock);
>>
>> + if (s->flags & SLAB_CONSISTENCY_CHECKS) {
>> + if (!validate_slab_ptr(slab)) {
>> + slab_err(s, slab, "Not a valid slab page");
>> + return NULL;
>> + }
>> + }
>> +
>> object = slab->freelist;
>> slab->freelist = get_freepointer(s, object);
>> slab->inuse++;
>>
>> - if (!alloc_debug_processing(s, slab, object, orig_size)) {
>> - if (validate_slab_ptr(slab))
>> - remove_partial(n, slab);
>> + if (!alloc_debug_processing(s, slab, object, orig_size))
>> return NULL;
>
> Is it intentional to not remove the slab from the partial list
> when alloc_debug_processing() returns false?

No, good catch, will fix. Thanks!

>> - }
>>
>> if (slab->inuse == slab->objects) {
>> remove_partial(n, slab);
>>
>> --
>> 2.51.0
>